Static cast molding of contact lenses involves casting a contact lens in a mold cavity formed by two mold sections, an anterior mold section including a molding surface for forming the anterior lens surface and a posterior mold section including a molding surface for forming the posterior lens surface. Representative static cast molding methods are disclosed in U.S. Pat. Nos. 5,271,875 (Appleton et al.), 4,197,266 (Clark et al.), 4,208,364 (Shepherd), 4,865,779 (Ihn et al.), 4,955,580 (Seden et al.), and 5,143,660 (Hamilton et al.).
Following casting of the lens, the mold assembly is disassembled and the lens is removed from the mold assembly.
One approach for recovering the contact lens involves exposing the entire mold assembly, including the two mold sections and the molded lens therebetween, to an aqueous solution (including water alone). Hydrophilic lenses absorb water to form a hydrogel, and when the lens absorbs water, it swells and is released from both the posterior and anterior molding surfaces of the respective molds. Often, the aqueous solution may also facilitate separation of the two mold sections from each other. The lens can then either be separated from the mold sections, either manually or with automated handling equipment. Representative "wet release" methods are discussed in U.S. Pat. No. 5,264,161 (Druskis et al.).
A second approach involves, initially, a "decapping" step, i.e., separating one mold section from the second mold section with the lens being retained in the second mold section. Then, the lens is removed (or released) from the second mold section, either by exposure to an aqueous solution (wet release) whereby the lens absorbs water to facilitate its separation from the molding surface of the mold section, or by simply removing the lens from the second mold section without the use of an aqueous solution (dry release).
Although the decapping and release operations may seem straightforward, various problems have been encountered. This is especially true for cast molding methods where the molding operation results in the two mold sections being held tightly together, or where the lens has a tendency to stick to one or both molding surfaces of the mold sections. For example, when the first mold section is decapped from the second mold section with the lens being retained in tile second mold section, the surface of the lens may be damaged as pieces of lens stick to the first mold section. Also, since the lens has not yet been hydrated, it is brittle, and the decapping operation can fracture the lens.
As another example of problems encountered, it is often desirable that the lens is selectively retained in a desired mold section to minimize manual handling or inspection. However, the decapping process can result in the lens not being retained in the desired mold section as intended, thus requiring manual handling or inspection to ensure that the lens is not discarded with the mold section removed in the decapping process. Various approaches for selective retention of the lens in a desired mold section have been proposed. U.S. Pat. No. 5,271,875 (Appleton et al.) discloses using mold sections made of different materials, however, this complicates the injection molding process. It is also known to treat one of the mold surfaces with plasma treatment to facilitate selective retention of the lens, however, this process adds additional steps and cost to the overall molding process.
Consider the case where it is desired to retain the molded lens on the anterior mold. A decapping operation of which applicant is aware involves decapping the posterior mold (i.e., separating the posterior mold from the anterior mold and molded lens) by holding the anterior mold firmly (for example, in a collet), followed by pulling the posterior mold or by pushing the posterior mold from the anterior mold (for example, with collet fingers). Applicant has found that this approach provides some success, however, yield is not entirely satisfactory due to damage to lenses during the decapping step, or lenses not being retained with the anterior mold as intended. Another approach would involve decapping the posterior mold by holding the posterior mold, followed by applying pressure to the anterior mold to separate the anterior mold and the lens from the posterior mold. However, this approach is less successful than the aforementioned approach in that there is a higher rate of lens damage and/or occurrence of the lens not being retained in the anterior mold.
The operations are further complicated by the fact that many static cast molding methods result in a ring of cured excess lens material being obtained in addition to the molded lens. Thus, while it is generally desirable that the lens is selectively retained in one desired mold section, as discussed above, it is often also desirable that the ring of cured excess lens material be selectively retained on the other mold section.